The present invention relates to methods and materials used for sealing the walls of a well bore passing through a porous formation during the drilling process for oil, gas and water wells in both on shore and offshore operations. The present invention is also useful in air drilling.
In general, during the drilling of oil wells, drilling fluid is passed continuously from the surface of the well bore to the bottom of the bore through the drilling bit and recycled back to the surface. The drilling fluid is introduced to lubricate the drill bit and drill string, cool the drill bit and additionally remove the cuttings from the bottom of the hole to the surface of the well bore. A major problem is encountered in the drilling process when the drill bit passes through porous or fractured formations wherein there is a high porosity which results in the drilling fluid being lost to the porous structure. This problem is commonly referred to as lost circulation. To either prevent or correct the problem of lost circulation, prior art workers have utilized a wide variety of substances as additives to the drilling fluid to prevent lost circulation. These materials are known as lost circulation materials. The lost circulation materials can be added during drilling or when a very large porous structure is encountered and the use of cement is required to seal off the porous structure, the lost circulation material may be pumped down the well prior to the addition of cement to form a binder or mesh surface to keep the amount of cement lost to the porous structure at a minimum.
The use of unginned cotton as a lost circulation material is described in U.S. Pat. No. 2,691,629 issued to Robert A. Stoner in 1954. In this patent, the addition of unginned cotton with the seeds attached was found to provide a useful lost circulation material. The patent additionally discloses in Tables 1 and 2 that according to tests performed on cottonseed hulls alone, adequate sealing was not accomplished by the use of cottonseed hulls alone.
The present invention discloses the heretofore unknown use of cottonseed hulls to provide an adequate lost circulation material without the necessity of using the bulky unginned cotton fibers. Since large quantities of the lost circulation materials are utilized in the process of drilling for oil and gas wells, it is very desirable to have a lost circulation material which is readily available, inexpensive, easily handled and readily mixed with the drilling fluid and easily pumped down the well bore. Many of the prior art materials such as leather, whole unginned cotton, wood fibers, etc. although inexpensive in their natural condition must be processed by grinding, fluffing or treating prior to use as a lost circulation material. The process of mixing can be awkward with a highly viscous drilling fluid resulting many times. The specially treated cottonseed hulls of the present invention are readily available in pelleted form as a feed commonly used for dairy cattle. The pellets are highly compressed material. Therefore, they are not as bulky as prior art materials and may be more easily handled. The more dense, less bulky material of the present invention is particularly desirable since in most drilling operations the space available for storing lost circulation material is limited, this is especially critical in off shore drilling operations. The pelleted shape allows easy addition of the cottonseed hulls by any convenient method such as a hopper. Because of their compressed nature and composition, the pelleted cottonseed hulls are easily mixed and pumped down the well bore without appreciably affecting the viscosity of the drilling fluid until they are in a position to seal off the porous structures. Additionally, the pelleted cottonseed hulls may be added directly to the well opening when desired.
It is particularly desirable to have such a lost circulation material which can be introduced into the well bore as quickly and easily as possible to keep lost circulation to a minimum when porous structures are encountered during the drilling process. The prior art materials are generally very fibrous and bulky in nature making introduction into the well bore difficult.
It is desirable to add as much lost circulation material to the drilling fluid as possible to seal off the porous structures in as short a time as possible. However, the amount of lost circulation material which can be added to a drilling fluid is limited due to the creation of a highly viscous drilling fluid which cannot be pumped down the well bore. The pelleted cottonseed hulls utilized in the present invention overcomes this problem by a novel time controlled expansion. The ingredients of the pelleted cottonseed hulls are heated and compressed during processing to prevent the expansion of the pellets during use as a lost circulation material until they are well down the drilling hole. This allows easy mixing without forming a viscous unpumpable drilling fluid followed at a later time by expansion of the pellets to exhibit the desired properties of a lost circulation material.